1. Field of the Invention
The invention relates to an optical signal generator for generating a multilevel-coded optical signal using an electro-absorption optical modulator.
2. Description of the Related Art
The optical communication system is required to achieve an increase in capacity for transmission information and an increase in transmission range, and in accordance with the requirement, the optical transmitter module is required to achieve a higher transmission rate and a lower chirp operation. The optical transmission module generates a modulated optical signal having been modulated based on a modulation signal representing the transmission information. As the modulation system, there can be cited a direct modulation system for directly modulating the optical intensity using a laser diode, and an external modulation system for modulating output light of a laser diode, and among these systems, the external modulation system is superior in chirp characteristics.
The optical transmitter module of the external modulation system may be constituted using an optical semiconductor element including a light emitting part and a modulating part integrated with each other. As the optical modulator used for the modulating part, there can be cited a Mach-Zehnder optical modulator (MZ modulator), an electro-absorption optical modulator (EA modulator), and so on.
The MZ modulator divides the wave of the input optical signal into two light paths and varies the intensity of the light using the interference between the two light paths. The MZ modulator can control the optical intensity of the multiplexed light by controlling the phase difference between the light paths.
The EA modulator has a structure of sandwiching a semiconductor layer (multiple quantum well (MQW) active layer) for forming a multiple quantum well by p-type and n-type semiconductor layers, and absorbs to quench the light using a phenomenon that the absorption edge wavelength of the light is shifted toward the long-wavelength side in accordance with the voltage applied to the quantum well (quantum confined Stark effect) to thereby control the intensity of the output light.
Since the MZ modulator has a structure of branching and merging the waveguide, in the case of forming the MZ modulator with a semiconductor element, the element size can be larger compared to the EA modulator. Further, a high dimensional accuracy is required for the formation of the waveguide. In contrast, the EA modulator is smaller in size and simpler in structure than the MZ modulator, and it is relatively easy to reduce the manufacturing cost.
As a technology of achieving an increase in capacity for transmission information and an increase in transmission rate, there can be cited a multilevel coding technology for expressing three or more levels with a single code such as a four-level pulse-amplitude modulation (PAM4) signal, and use of the technology has been studied also in the optical communication system. In Japanese Translation of PCT International Application Publication No. JP-T-2010-507131, for example, there is described a technology of performing multilevel coding using the MZ modulator.